Cottonseed meal and oil made by the extraction of partially dehulled cottonseed with hexane are widely used. However, cottonseed oil is difficult to refine; and cottonseed meal often sells at a disadvantage compared with soybean meal, and at times cannot be used at all. There are two problems:
(1) Cottonseed may be contaminated with aflatoxin. At the present time there is no commercially accepted process for removing or detoxifying aflatoxin.
(2) Cottonseed contains gossypol. This is a toxic pigment that must be either removed from or chemically bound in the meal.
In present practice, cottonseed is either prepressed and extracted with hexane, or extracted without prepressing. In the prepress method, cottonseed meats are moistened and heated before pressing to promote flow of the oil from the press cake. Under such conditions, the gossypol glands rupture, releasing into the oil much of the gossypol which is difficult to refine from the oil. The remainder of the gossypol is bound chemically in the meal.
For direct extraction, cottonseed is partially duhulled and flaked after little or not heating. Such preparation and subsequent extraction with hexane do not rupture the gossypol glands. The oil-free meats are then deliberately subjected to high moisture and temperature so as to rupture the glands. Under these conditions the gossypol reacts with and becomes chemically bound in the meal.
Neither of these processes is completely satisfactory. In both, the protein of the meal is badly denatured. Oil from prepressing is difficult and expensive to refine. Such pigments as diffuse from the meals into the miscella from hexane extraction are difficult to remove from the oil after the oil is desolventized. The pigments are best removed by refining undistilled or partly distilled miscella, an expensive process. Although retention, in direct extracted meals, of substantially all of the gossypol originally in the seed is not tolerated, it is generally agreed that so high a gossypol concentration is undesirable, even though the gossypol is bound.
It is known that aqueous alcohols are potentially good solvents for treating cottonseed. Relatively dilute aqueous alcohols dissolve aflatoxin, gossypol, fatty acids and non-oil lipids. Cottonseed oil has only limited solubility in cold concentrated alcohols or in hot dilute alchols.
It is known that aflatoxin can be extracted from full-fat cottonseed or hexane-extracted cottonseed meats with hot aqueous ethanol or isopropanol at concentrations stronger than 80 weight percent. However, this has not resulted in a commercial process for aflatoxin extraction. No process is available for extracting oil with aqueous alcohols following extraction of aflatoxin from full-fat cottonseed with aqueous alcohols. Extraction of aflatoxin with an aqueous alcohol from oil-free meats produced by hexane extraction is deemed too expensive.
It is known that gossypol glands are ruptured by aqueous alcohol solutions, and that gossypol is soluble in such solutions. When the temperature at which the glands are ruptured is low (below 140.degree. F.), the gossypol remains largely in solution; as the temperature is increased, the gossypol is rapidly bound in the meal. However, there is no commercial process available by which such rupture and solubility is taken advantage of to remove gossypol.
In U.S. Pat. Nos. 4,144,229 and U.S. Pat. No. 4,219,470, assigned to the same assignee as the present invention, there are disclosed processes for extracting carbohydrates and non-oil lipids from oilseeds, particularly soybeans, leaving a residue of high-protein flour or concentrate. In such processes, there are generally four sequential steps in which the oilseed is countercurrently treated with aqueous solutions of a monohydric alcohol, such as ethanol or isopropanol:
In the first step, carbohydrates and non-oil lipids are extracted with dilute alcohol. For example, if the alcohol is ethanol, a preferred concentration is 50-70 weight percent. At such ethanol concentration, carbohydrates and non-oil lipids are selectively extracted; protein and oil are only sparingly soluble.
In the second step, the dilute alcohol in the seed material from the first step is displaced with aqueous alcohol of maximum feasible concentration. If the alcohol is ethanol, the maximum concentration that can be made by practical distillation without special means is about 92 weight percent. The concentration of the ethanol available for displacing the dilute ethanol in the second step is about 90-91 weight percent.
In the third step, the seed material from the second step is further extracted with concentrated alcohol at or near its boiling point. Under such conditions, oil is sufficiently soluble to make feasible a process in which oil-saturated miscella from the third step is cooled to precipitate oil, and the lean miscella is recycled to the third step extraction. The amount of miscella circulating through the third step must be at least sufficient to dissolve all of the oil that enters with the seed material.
In the fourth step, extraction of the seed material leaving the third step, which is still not completely free of oil, is completed by contacting the seed material with distilled alcohol, 92 weight percent if ethanol.
It is an important advantage of the four-step process outlined above that the oil produced in the third step is semi-refined. This is attributed to the removal in the first step of carbohydrates and non-oil lipids which otherwise would precipitate with the oil in the third step.
A protein flour or concentrate and semi-refined oil can be made from full-fat cottonseed by this process. However, there is a need for a process that can produce cottonseed meal, with unenhanced or only slightly enhanced protein content, that is free of aflatoxin, is free of unbound gossypol, has a minimum of bound gossypol and has protein of high solubility, while producing semi-refined oil.
In considering a four-step process for extracting cottonseed with minimum extraction of carbohydrates, there was no basis for estimating the maximum alcohol concentration permissible in the first step which would remove enough carbohydrates and non-oil lipids to assure production of semi-refined oil in the third step. The equivalent of 70 weight percent ethanol would certainly extract too much carbohydrates in the first step; omission of the first step entirely would likely result in poor oil.